The earth’s surface is dominated by water, but only about 2.5% of the estimated 1.4 billion cubic kilometres is fresh water. Most fresh water (68.9%) is stored as snow and ice in glaciers and icecaps, while most of the rest (30.8%) is groundwater. Only 0.3% of fresh water is in rivers and lakes.
New Zealand has abundant water resources in snowfields, glaciers, groundwater aquifers, rivers and lakes. It ranks in the top 10 countries in the world for both quantity and quality of water.
Despite this abundance, the country is facing growing challenges in managing its water. In the 2000s, demand for water was increasing dramatically as a result of urban expansion and agriculture. Water supplies were not always in the right places at the right times to meet these demands.
When demand exceeds supply – as is happening increasingly in drier, eastern parts of New Zealand – competition for water is fierce. These demands also peak during summer, when the water level is generally at its lowest. Climate change makes management of water resources even more complicated, as rainfall and snowfall patterns vary.
Too much water (floods) or too little water (droughts or low river flows) can both be a problem, and can damage property, livelihoods, roads and bridges.
Knowing how often low flows are likely to occur is important for river water users such as irrigators, town water suppliers and hydroelectric generators. It can also help protect the ecosystem.
Similarly, knowing likely flood patterns aids with designing bridges, and protection works such as stopbanks and detention dams.
Return periods for floods (of a certain size) and droughts (of a certain duration) are often expressed as a probability, based on historic records. For example, a one-in-100-year flood event means that, on average, a flood of that size will occur once every 100 years.
Annual extremes are analysed to estimate the probability of a drought or a flood.
Climate change is expected to affect the frequency of extreme weather events. For example, predicted changes in rainfall patterns on the South Island’s east coast are likely to increase the frequency of droughts – by 2080 droughts that currently occur about once in 20 years could come once in five years.
Scientists are working to increase the warning period for floods, so impacts on infrastructure, stock and people can be reduced.
Flood models help predict when floods occur in relation to rainfall patterns, and the movement of flood peaks (the highest level of the flood) down through the catchment. Models also predict which areas are likely to be flooded, and can help in designing and planning flood control work.
Floods and droughts are New Zealand’s most damaging and costly natural hazards. For example, the 1984 Southland floods and 2004 Manawatū floods are estimated to have cost the economy over $100 million each. The Canterbury drought of 1987–88 had an estimated cost of $360 million, while the 1998–99 Otago drought probably cost the economy $600 million.
The hydrological cycle describes the movement of water in the environment. Water is constantly moving, and is easily transformed from ice to water to vapour.
The sun’s energy drives the hydrological cycle. It evaporates water from oceans, other water bodies and plants. Water vapour returns to earth as rain, hail, fog, sleet and snow. Water moves downhill through glaciers, streams, aquifers (underground water), rivers and lakes. It moves at differing speeds, depending on whether it is in a solid or liquid state, and on the landscape through which it drains. For example, glacier ice moves very slowly – an increase in snowfall at the head of the glacier can take 5–8 years to be reflected in the position of the glacier face. In contrast, small streams can rise within minutes of heavy rainfall.
A water balance study explains and quantifies the hydrological cycle for a particular region or catchment, by measuring various components of the cycle. Some components, such as rainfall, stream flow and groundwater level, are relatively easy to measure. Others, such as water vapour in clouds, evapotranspiration (water loss from plants and soil) and soil moisture, are more difficult to measure.
An added complexity for these studies is that measurements need to be adjusted to account for New Zealand’s mountainous terrain, which makes it problematic to use rainfall recordings from one rain gauge to characterise rainfall over the entire river catchment, which could be 100 to 1,000 square kilometres in size. Also, rainfall patterns can vary dramatically over relatively small areas of a catchment. For example, during floods in Manawatū in 2004, some parts of the catchment received downpours while other parts had very little rain.
The hydrological cycle is strongly influenced by topography, geology, soils and vegetation. However, it can also be modified through human activities, such as taking water for power generation, industry, agriculture and water supply; discharges into rivers; construction of dams; changes in vegetation patterns (such as planting pine forests); and changes in the shape of the riverbed. For example, streams in urban areas often swell rapidly after rainfall because of stormwater drainage from impermeable surfaces such as roads or car parks.
At times of low flow, urban streams also tend to dry up more quickly than non-urban streams. Paved surfaces in urban areas reduce the amount of water seeping into groundwater, much of which would normally then be slowly released into rivers or streams. Groundwater often makes a major contribution to the flow of a river or stream.
Water resources are managed to minimise human impacts on the natural hydrological cycle.
In New Zealand, water is managed by catchment, so regional councils are able to concentrate on the entire contributing area for a whole river catchment or aquifer. It is rare for water to be transferred from one catchment to another. However, one example is the Tongariro power scheme, which takes water from the Whanganui River catchment and diverts it to the Waikato catchment.
In any given year, around 500 billion cubic metres of fresh water falls as precipitation throughout New Zealand. New Zealand receives most of its precipitation as rain, and the rest as snow, plus occasional hail and sleet.
While New Zealand has abundant rainfall, it is not always in the places it is needed for irrigation, power generation, or domestic supply.
The Southern Alps have some of the highest annual rainfalls in the world – on average, more than 10,000 millimetres each year. This is caused by the moist airflow from the Tasman Sea being blocked by the mountains.
The Southern Alps cause a rain-shadow effect to their east. Remarkably, annual rainfall in Central Otago, within 100 kilometres of the alps, can be as low as 400 millimetres per year.
North Island rainfall tends to be around 1,000 millimetres per year, with less in the eastern regions, and more in the west and north. Because the North Island is less mountainous than the South Island, rainfall levels tend to be more even across the island.
Snowfall occurs mainly in the alpine areas, but occasionally down to sea level. Most seasonal snow in New Zealand is concentrated in the Southern Alps and on the volcanic cones of the North Island. In winter, up to 35% of the South Island is snow-covered. The permanent snowline is above 1,500–2,200 metres.
In alpine areas, river flows reduce in winter when snow falls, and increase during spring and summer thaws. Many hydroelectric schemes rely on snowmelt to fill storage lakes.
A glacier is a mass of snow and ice. New Zealand has about 3,140 glaciers – only 18 are in the North Island, and all of those are on Mt Ruapehu. The Tasman Glacier near Aoraki/Mt Cook is New Zealand’s largest, with an area of nearly 10,000 hectares. Almost 50% of all New Zealand’s ice is contained in the 10 largest glaciers.
Fluctuations in glacier ice provide an excellent tool for measuring climate variability, as changes in glacier mass directly reflect the climate in the preceding year.
New Zealand glaciers, like those worldwide, have been shrinking since the late 1800s. Around 25% of New Zealand’s ice cover has been lost over the last 150 years.
A considerable amount of New Zealand’s fresh water is stored as groundwater in aquifers – underground layers of water-soaked rock or gravel.
Confined aquifers have an impermeable layer of rock or sediment above and below them; unconfined aquifers only have an impermeable layer below. Although they are deeper than unconfined aquifers, confined aquifers can store water under pressure, which can make extraction easier once a well has been drilled.
Aquifers can lie under any landscape, but those under relatively flat land (such as broad river flood plains) tend to have more water and be more accessible. For example, beneath the Canterbury Plains are layers of gravel aquifers containing a great deal of water from rainfall on mountains, foothills and plains, plus leakage from rivers. The water moves slowly eastward down the plains to the sea.
Aquifers are fed by rainfall and rivers. They also flow back into rivers and, on the lower plains, are the source of spring-fed rivers such as the Avon River in Christchurch.
Around one-third of the water used in New Zealand comes from groundwater. New Zealand’s groundwater storage has been estimated as 612 billion cubic metres.
Groundwater has been used as a source of high-quality drinking water since early European settlement, and is the source of the domestic water supply for cities such as Christchurch and Napier. Careful management of the source aquifers and their recharge zones (areas where water goes underground) is required to maintain this high quality.
Groundwater for Christchurch’s town supply is some of the purest in the world. Drawn from aquifers that are 20–200 metres deep, it is thought that some of it has been underground for as long as 800 years. Some of it is under pressure – a bore driven in 150 metres creates a 9-metre-high fountain.
New Zealand has more than 50,000 lakes, although fewer than 4,000 are more than 1 hectare in area. Lakes have an important role in water distribution as they tend to store river water and slow down the transport of water, solutes (dissolved substances) and sediments to the sea.
New Zealand is dotted with storage reservoirs (artificial lakes, or natural lakes with raised water-levels), ranging in size from small farm dams, through to the 7,500-hectare Lake Benmore.
Many of the larger reservoirs have been created for generating hydroelectricity. There are three main hydroelectric storage lakes in New Zealand, which hold 70% of the water used to generate electricity. Lake Pūkaki has the largest storage (around 35% of the national storage), followed by nearby Lake Tekapo (around 21%) and Lake Taupō (14%).
Hydroelectricity provides 70% of New Zealand’s electricity generation capacity. Hydroelectric power generation uses around 114 cubic metres of water per person per day. All of this water is returned to natural systems. However, dams are not without ecological impacts, as they trap sediment, stop the passage of fish and alter downstream flow.
Storage reservoirs are also used for irrigation, as they allow water to be harvested during times of plenty and released at times most beneficial to farmers. They can also provide other benefits – a report showed that Lake Ōpuha (an irrigation reservoir created by damming the Ōpuha River in 1999) provided 480 jobs and an extra $124 million per year to South Canterbury’s economy. It also has provided stable minimum flows in the Ōpihi River, sustaining a valued trout fishery.
New Zealand has around 426,000 kilometres of rivers. In terms of the quantity of water discharged, South Island rivers hold the top five positions, and only two North Island rivers make the top 10 (Waikato and Whanganui). Four drain to the Tasman Sea (Buller, Grey, Taramakau and Haast) – these rivers are relatively short but carry enormous volumes of water from the heavy rain band along the Southern Alps.
The flow regime of a river is the way its flow changes from day to day, season to season and year to year. Periods of low flow are interspersed with floods, caused by storm rainfall upstream in the river catchment.
In New Zealand, low flows tend to occur in summer when rainfall is usually lower, water loss from plants and soil is higher due to evaporation, and soil moisture and groundwater levels decrease.
The amount of rainfall required to produce a flood in summer is usually greater than in winter, when the catchment tends to be wetter and so is more strongly affected by storm rainfalls.
In alpine areas, low-flow periods occur more often in winter, because precipitation is stored in the catchment as snow and ice, which is released during spring and summer thaws.
Taking water for domestic and agricultural uses has led to many small rivers in drier eastern parts of New Zealand – such as the Pareora in South Canterbury – becoming shadows of their former selves. Communities need to weigh up the economic gains from taking water out of natural systems against the ecological and recreational costs – such as not having swimming holes and trout to catch.
New Zealand has a huge variety of river types, including boulder-filled mountain torrents, braided rivers on coastal plains, meandering lowland spring-fed channels and concrete-lined urban waterways. Each river’s character is a product of climate, catchment geology, water source and vegetation characteristics.
The Ministry for the Environment and the National Institute of Water and Atmospheric Research developed the River Environments Classification system, to aid the monitoring and management of rivers. The system groups rivers of similar physical characteristics, so they can be compared with one another.
Rivers can be immensely powerful agents of change in the landscape, through flooding and changing path. To protect against this, river engineers have, in many places, modified riverbeds and banks to limit potential floods and control the river’s path. For example, around 1,000 years ago the Waimakariri River flowed through the current location of Christchurch. Without extensive stopbanks on the lower Waimakariri and gravel extraction on the riverbed, changes in the course of the river during floods might have catastrophic consequences for Christchurch.
Managing water resources means grappling with questions such as who has rights to the available water, how to manage those rights, and how to protect the environment.
The Resource Management Act 1991 requires regional councils to manage water resources in a sustainable manner. A major problem for councils is defining standards to measure environmental conditions – for example, if the water in a particular river dropped below a certain level then no more water should be taken. However, there has been uncertainty over how to decide what such standards should be, and who should set them.
From the 1990s and into the 2000s, increased demand for water and decreased water quality has shown a need for better management of water resources. Regional councils called for greater leadership from central government. In 2003 government responded with the Sustainable Water Programme of Action, which is coordinated by the Ministry for the Environment and the Ministry of Agriculture and Forestry.
The programme aims to deal with increasing demand for water by encouraging efficient water management, working with local government and communities, and developing standards. National policy statements provide direction, and national environmental standards allow monitoring.
Regional councils, power generation companies, GNS Science and the National Institute of Water and Atmospheric Research maintain an extensive network of climate and water monitoring sites throughout the country.
Water allocation means dividing available water resources across competing or conflicting uses. In New Zealand, there is a ‘first come, first served’ principle. The water allocation process is defined in regional council policies and plans. Decisions are made through resource consent hearings and rules in district plans.
In some rivers, water allocation limits have been determined by water conservation orders, which aim to preserve specific water bodies in their natural state (or as close as possible to it) to protect recreational or natural features. In 2008 there were 14 water conservation orders, with the most recent (2006) providing protection for the Rangitātā River in South Canterbury.
Almost 80% of the water extracted in New Zealand is used for irrigation for viticulture, horticulture, dairying, beef and sheep farming, and cropping. There is approximately 500,000 hectares of irrigated land in New Zealand, almost one-third of which is used for dairying. In recent years, dairying has expanded into drier areas such as Canterbury, requiring considerable amounts of water for irrigation.
The annual value of irrigation water in terms of increased agricultural production is estimated to be around $800 million.
Irrigation has become widespread on the dry east coast of the South Island, but is not new. Duntroon farmer Sid Hurst recalled that ‘he has always known what water could do for farmers on the Waitaki plains. As a young lad he helped siphon water illegally out of the Oamaru Borough race to wild flood paddocks. The response was dramatic. Irrigated areas turned green and flourished, creating a stark contrast with the arid brown of surrounding paddocks.’ 1
Removing water from a river obviously reduces the quantity that will be flowing downstream. It can also reduce water quality, as there is less water to flush away solutes (dissolved substances) and sediments. Rivers can also flow more slowly, and become shallower and warmer, making it easier for algae to grow and less suitable for fish and other aquatic life.
One of the major issues affecting water resources management is the limited information available on volumes of water actually taken. Good records of water allocation are kept but there has often been either more or less water used than what was actually allocated.
A new national environmental standard for water-measuring devices will greatly improve measurements of how much water is actually taken.
Harding, Jon, and others, eds. Freshwaters of New Zealand. Wellington: New Zealand Hydrological Society and New Zealand Limnological Society, 2004.
Mahon, Sam. The water thieves. Dunedin: Longacre Press, 2006.
Mosley, M. Paul, ed. Waters of New Zealand. Wellington: New Zealand Hydrological Society, 1992.
Mosley, M. Paul, and Charles P. Pearson, eds. Floods and droughts: the New Zealand experience. Wellington: New Zealand Hydrological Society, 1997.
Parkinson, Brian, and Geoffrey Cox. A field guide to New Zealand’s lakes and rivers. Auckland: Random Century, 1990.
Young, David, and Bruce Foster. Faces of the river: New Zealand’s living water. Auckland: TVNZ Publishing, 1986.